1. Field of the Invention
This invention deals generally with the simultaneous generation of short pulses at multiple wavelengths, which is of particular interest to wavelength-division-multiplexed communication systems, photonic microwave systems, and pump-probe applications. A versatile and cost-effective laser source should have the following characteristics. Synchronized multiple-wavelength outputs eliminate the need for external synchronization when using more than one wavelength. Short pulses of less than 10 ps are needed for gigahertz repetition rates. The instant invention is a multiple-wavelength laser based on the sigma configuration, well known in the prior art, which has tunable wavelength, synchronized outputs, short pulses, and low noise.
2. Description of the Related Prior Art
Mode-locked pulse generation at multiple-wavelengths has been studied in various configurations of erbium-doped fiber lasers. The focus has primarily been on variations in the ring cavity design. Schlager, et al., have proposed a method that incorporates 3.4 km of birefringent fiber and a polarizing isolator in the cavity (See J. B. Schlager, S. Kawanishi, and M. Saruwatari, “Dual wavelength pulse generation using mode-locked erbium-doped fiber ring laser,” Electron. Lett., 27, 2072-2073 (1991)). Other techniques for selecting the wavelength for ring cavity lasers utilize fiber Bragg gratings, power splitters and bandpass filters, and dense-wavelength-division-multiplexers. Passively modelocked lasers are demonstrated in a figure-eight configuration by Noske, et al. and in a linear configuration by Okhotnikov, et al. (See D. U. Noske, M. J. Guy, K. Rottwitt, R. Kashyap, and J. R. Taylor, “Dual-wavelength operation of a passively mode-locked “figure-of-eight” ytterbium-erbium fiber soliton laser,” Opt. Commun., 108, 297-301 (1994) and O. G. Okhotnikov and M. Guina, “Stable Single- and Dual-Wavelength Fiber Laser Mode Locked and Spectrum Shaped by a Fabry-Perot Saturable Absorber,” Opt. Lett., 25, 1624-1626 (2000)). An actively-modelocked linear cavity laser with fiber gratings as the ends of the cavity and a nonlinear optical loop mirror in the middle has been demonstrated by Pattison et al. (See D. A. Pattison, P. N. Kean, J. W. D. Gray, I. Bennion, and N. J. Doran, “Actively Modelocked Dual-Wavelength Fiber Laser with Ultra-Low Inter-Pulse-Stream Timing Jitter,” Photon. Technol. Lett., 12, 1415-1417 (1995)) A modified sigma configuration laser, which used a circulator followed by cascaded fiber gratings in a second arm is demonstrated by Deparis, et al. (See O. Deparis, R. Kiyan, E. Salik, D. Starodubov, J. Feinberg, O. Pottiez, P. Megret, and M. Blondel, “Round-trip time and dispersion optimization in a dual-wavelength actively mode-locked Er-doped fiber laser including nonchirped fiber bragg gratings,” IEEE Photon. Technol. Lett., 11, 1238-1240 (1999)). These lasers produce either broad pulse widths (>10 ps) or wide wavelength separations (>15 nm). This limits the ability of the laser to produce high repetition rates and limits the total number of wavelengths that are possible within the gain bandwidth of the amplifying media. Additionally, wide wavelength tuning ranges have not previously been demonstrated.